Method of interference cancellation and method of detection of erroneous neighbour cell measurements

ABSTRACT

Methods of interference cancellation are provided. Channel estimation is performed with or without interference cancellation. Methods of detection of erroneous neighbor cell measurements are provided. The channel estimates for neighbor cells are processed to identify unreliable measurements.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/472,023 filed Apr. 5, 2011 and U.S. Provisional Application No.61/496,355 filed Jun. 13, 2011 both hereby incorporated by reference intheir entirety.

FIELD

The application relates to interference cancellation, and to methods ofdetecting erroneous neighbor cell measurements.

BACKGROUND

In some existing systems, such as in TD-SCDMA (Time Division SynchronousCode Division Multiple Access), HCR (High Chip Rate Time DivisionDuplex) and VHCR (very HCR Time Division Duplex) systems, interferencecancellation is used to cancel a serving cell's/other strong cell'ssignals from an overall signal for the purpose of preparing to measurereceive power of a cell's signals, and in particular, for measuring ofthe RSCP (Received Signal Code Power) of the P-CCPCH (Primary CommonControl Physical CHannel).

Regardless of whether interference cancellation is used or not thepresence of (remaining) interference is a serious concern in CDMAsystems, in particular the effect of such interference on the aforesaidmeasurements. The issue might arise that the remaining interference iscovering up the measurement result such that it might become unusable.

SUMMARY

According to one aspect of the present application, there is provided amethod comprising: processing a signal to produce an interferencecancellation component of a first cell; performing signal detection fora to be detected cell operating on a same frequency as the first cellbased on the signal minus the interference cancellation component if theinterference cancellation component has a power that is large enoughcompared to a total power of the signal as defined by a first threshold;performing signal detection for a to be detected cell based on thesignal without having subtracted the interference component if theinterference cancellation component has a power that is not large enoughcompared to the total power of the signal as defined by the firstthreshold.

In some embodiments, there is provided the method as summarized abovewherein processing the signal to produce the interference cancellationcomponent comprises: generating a channel estimate for the first signalusing a cell-specific code of a first cell, the channel estimatecomprising a plurality of taps; removing certain taps from the channelestimate to produce a post-processed channel estimate; using thecell-specific code and the channel estimate with the certain tapsremoved to reconstruct the interference cancellation component; whereinperforming signal detection of a to be detected cell based on the signalminus the interference cancellation component comprises using acell-specific code of the to be detected cell; and performing signaldetection of a to be detected cell based on the signal without havingsubtracted the interference component comprises using the cell-specificcode of the to be detected cell.

In some embodiments, there is provided the method as summarized abovewherein removing certain taps comprises removing taps that each have apower that is small enough compared to the total power of the signal asdefined by a second threshold.

In some embodiments, there is provided the method as summarized abovewherein each of the taps other than the certain taps has a power that islarge enough compared to the average power of taps other than thecertain taps as defined by a second threshold.

According to another aspect of the present application, there isprovided a method comprising: processing a signal to produce a channelestimate in respect of a first cell. the channel estimate comprising aplurality of taps; removing certain taps from the channel estimate toleave remaining taps and then producing an interference cancellationcomponent from the remaining taps; performing first signal detection ofa to be detected cell operating on a same frequency as the first cellbased on the signal minus the interference cancellation component toproduce a first signal detection result; performing second signaldetection of the to be detected cell based on the signal without havingsubtracted the interference cancellation component to produce a secondsignal detection result; selecting between the first signal detectionresult and the second signal detection result.

In some embodiments, there is provided the method as summarized abovewherein: performing first signal detection comprises performing channelestimation to produce a first channel estimate comprising a plurality oftaps and removing certain taps to produce a first post-processed channelestimate; performing second signal detection comprises performingchannel estimation to produce a second channel estimate comprising aplurality of taps and removing certain taps to produce a secondpost-processed channel estimate; selecting between the first signaldetection result and the second signal detection result is based on: thepower of taps in the first post-processed channel estimate; the power oftaps in the second post-processed channel estimate; the average power oftaps in the first channel estimate; and the average power of taps in thesecond channel estimate.

In some embodiments, there is provided the method as summarized abovewherein selecting comprises: for the first signal detection, determininga ratio of a sum of powers of the taps remaining after removing certaintaps to the average power of taps of the first channel estimate; for thesecond signal detection, determining a ratio of a sum of powers of thetaps remaining after removing certain taps to the average power of tapsof the second channel estimate; selecting the result with the largerratio.

According to still another aspect of the present application, there isprovided a method comprising: processing a signal to produce aninterference cancellation component that is an estimate of a componentof the signal that is due to a first cell by: generating a channelestimate for the first signal using a cell-specific code of a first cellto produce a channel estimate comprising a plurality of taps; removingcertain taps from the channel estimate to produce a post-processedchannel estimate; producing the interference cancellation componentusing post-processed channel estimate; performing channel estimation ofa to be detected cell operating on a same frequency as the first cellbased on the signal minus the interference cancellation component by:generating a channel estimate for the to be detected cell using acell-specific code of the to be detected cell; removing certain tapsfrom the channel estimate to produce a channel estimate with certaintaps removed for the to be detected cell; the method further comprisingat least one of: a) discarding the channel estimate with certain tapsremoved for the to be detected cell and/or reporting a lowest reportablevalue and/or processing a very small value and/or not reporting thechannel estimate if a combined power of the taps of the channel estimatewith certain taps removed that define a to be measured channel for theto be detected cell is below a total receive power for the signal by athreshold amount; and b) discarding the channel estimate with certaintaps removed for the to be detected cell and/or reporting a lowestreportable value and/or processing a very small value and/or notreporting the channel estimate if a combined power of the taps of thechannel estimate with certain taps removed for the first cell is smallenough compared to a total receive power for the signal as defined by athreshold amount.

According to yet another aspect of the present application, there isprovided a method of processing a signal, the method comprising:obtaining a channel estimate for a cell, the channel estimate comprisinga plurality of taps; removing certain taps from the channel estimate; ifa combined power of taps of the channel estimate after removing certaintaps that define a to be measured channel is less than a certainthreshold below an amount based on a total power of the signal,discarding the channel estimate for the cell and/or reporting a minimumreportable receive power and/or processing a very small value.

In some embodiments, there is provided the method as summarized abovewherein removing certain taps from the channel estimate comprises usinga regression approach to differentiate between the taps to be removedand the taps not to be removed.

In some embodiments, there is provided the method as summarized abovewherein using a regression approach comprises: sorting the plurality oftaps into a sorted list; performing a regression on a subset of taps ina sorted list representing interference and noise to produce aregression result; removing taps that are not large enough compared tothe regression result as defined by a threshold.

In some embodiments, there is provided the method as summarized abovewherein the certain taps comprises taps that each have a power that issmall enough compared to the total power of the signal as defined by asecond threshold.

In some embodiments, there is provided the method as summarized abovewherein taps other than the certain taps are those that each have apower that is large enough compared to the average power of the certaintaps as defined by a second threshold.

In some embodiments, there is provided the method as summarized above,comprising: without first performing interference cancellation, a mobiledevice processing a signal to detect a ghost cell, a ghost cell being acell with an unreliable measurement result as defined by ghost celldetection criteria; in case a ghost cell is detected, the mobile devicesystematically searching for cells and their midamble codes which arenot in a neighbor cell list; for each cell found as a result of thesystematic search, the wireless device applying interferencecancellation of a component of the received signal due to the cell if atotal received power as defined by a sum of taps of a post processedchannel estimate of the cell is large enough as defined by a thresholdcompared to the overall received power of the midamble.

According to a further aspect of the present application, there isprovided a method comprising: without first performing interferencecancellation, a mobile device processing a signal to detect a ghostcell, a ghost cell being a cell with an unreliable measurement result asdefined by ghost cell detection criteria; in case a ghost cell isdetected, the mobile device systematically searching for cells and theirmidamble codes which are not in a neighbor cell list; for each cellfound as a result of the systematic search, the wireless device applyinginterference cancellation of a component of the received signal due tothe cell if a total received power as defined by a sum of taps of a postprocessed channel estimate of the cell is large enough as defined by athreshold compared to the overall received power of the midamble.

In some embodiments, there is provided the method as summarized above,comprising: processing a signal to detect a cell in accordance with aghost cell detection criteria, a ghost cell being a cell with anunreliable measurement result as defined by the ghost cell detectioncriteria; in case a cell is detected with the ghost detection threshold,determining that the cell is a ghost cell if the cell's BCCH (broadcastcontrol channel) cannot be detected; determining that the cell is not aghost cell if the cell's BCCH can be detected successfully.

According to still a further aspect of the present invention, there isprovided a method comprising: processing a signal to detect a cell inaccordance with a ghost cell detection criteria, a ghost cell being acell with an unreliable measurement result as defined by the ghost celldetection criteria; in case a cell is detected with the ghost detectionthreshold, determining that the cell is a ghost cell if the cell's BCCH(broadcast control channel) cannot be detected; determining that thecell is not a ghost cell if the cell's BCCH can be detectedsuccessfully.

In some embodiments, there is provided the method as summarized abovefurther comprising: where interference cancellation is employed inrespect of a primary-common control channel of a to be cancelled cell,the method further comprising performing interference cancellation of atleast one other channel transmitted by the to be cancelled cell.

In some embodiments, there is provided the method as summarized abovewherein performing interference cancellation for at least one otherchannel known to be transmitted by the to be cancelled cell comprisesperforming interference cancellation for at least one of S-CCPCH, PICH,FPACH (Secondary-Common Control Channe, Paging Indicator Channel, FastPhysical Access Channel).

In some embodiments, there is provided the method as summarized abovefurther comprising: determining an overall receive power; determining apower associated with at least one midamble shift other than a midambleshift of interest, the at least one other midamble shift using the samebasic midamble code as the midamble shift of interest; subtracting thepower associated with the at least one midamble shift other than themidamble shift of interest from the overall receive power to produce acorrected total receive power, and using the corrected total receivepower in place of the overall receive power.

In some embodiments, there is provided the method as summarized abovefurther comprising: for a to be cancelled cell, determining the at leastone midamble shift other than the midamble shift of interest frombroadcast system information.

In some embodiments, there is provided the method as summarized abovefurther comprising: for a to be cancelled cell, determining the at leastone midamble shift other than the midamble shift of interest fromalready known behaviour of another nearby cell, based on an assumptionthat nearby cells will behave similarly.

In some embodiments, there is provided the method as summarized abovefurther comprising adjusting the power associated with the at least onemidamble shift other than the midamble shift of interest that issubtracted from the total receive power to produce a corrected totalreceive power by: determining a set of taps for the P-CCPCH(Primary-Common Control Channel); determining the power associated withthe least one midamble shift using only taps that belong to the set oftaps for the P-CCPCH.

In some embodiments, there is provided the method as summarized abovefurther comprising: determining a receive power associated with unusedchannel estimation windows; adjusting the power associated with the atleast one midamble shift other than the midamble shift of interest thatis subtracted from the total receive power to produce a corrected totalreceive power by: setting the power to zero if it is not larger than thepower associated with the unused channel estimation windows by athreshold amount.

In some embodiments, there is provided the method as summarized abovefurther comprising: determining an average receive power associated withunused channel estimation windows; discarding the channel estimate forthe cell and/or reporting a minimum reportable receive power and/orprocessing a very small value if it is not larger than the powerassociated with the unused channel estimation windows by a thresholdamount.

In some embodiments, there is provided the method as summarized abovefurther comprising: determining if a cell measurement is reliable orunreliable in accordance with a criteria that spans over multiplemeasurement intervals.

In some embodiments, there is provided the method as summarized abovefurther comprising: processing a signal to detect a cell in accordancewith a ghost cell detection criteria, ghost cell being a cell with anunreliable measurement result as defined by the ghost cell detectioncriteria; looking systematically for cells that are not included in aneighbor cell list, and if some are found, treating them as known cells,for the purpose of performing interference cancellation; at least oneof: if no such cell is found found, then declaring the cell detected inaccordance with the ghost detection criteria to be a ghost cell; if nosuch cell is found, attempting to detect a BCCH of the cell, anddeclaring the cell detected in accordance with the ghost detectioncriteria to be a ghost cell if the BCCH cannot be detected.

In some embodiments, there is provided the method as summarized abovefurther comprising: assigning one of two states to a cell, the twostates being reliable_cell or ghost_cell; transitioning between the twostates on the basis of a criteria applied to multiple consecutivemeasurements.

In some embodiments, there is provided the method as summarized abovefurther comprising: recognizing when it is difficult to synchronize to acell, and using such information to aid the identification of a ghostcell.

In some embodiments, there is provided the method as summarized abovefurther comprising: when a ghost cell is detected with a high totalreceive power and no interference cancellation being performed,performing a systematic search for cells not included in a neighbor celllist, and if some are found treating them as known cells for the purposeof interference cancellation.

In some embodiments, there is provided the method as summarized abovefurther comprising: attempting to read the BCCH of a cell in theneighbour cell list; in case the SNR on a carrier of the cell on theneighbour cell list would not allow the BCCH of the cell in the neighborcell list to be detected with sufficient quality this cell is either notreported or reported at the option of the mobile device for neighborcell measurements not being in need to be reported or reported with theminimum reportable RSCP for neighbor cell measurements which have to bereported.

In some embodiments, there is provided the method as summarized abovefurther comprising: estimating an SNR of a to be detected/measured cell;in case the SNR on that carrier would not allow the BCCH of the cell inthe neighbor cell list to be detected with sufficient quality this cellis either not reported or reported at the option of the mobile devicefor neighbor cell measurements not being in need to be reported orreported with the minimum reportable RSCP for neighbor cell measurementswhich have to be reported.

In some embodiments, there is provided the method as summarized abovefurther comprising: using the input of the channel estimation for afirst cell as an input to performing channel estimation for a secondcell.

In some embodiments, there is provided the method as summarized abovewherein the method is performed in a user equipment.

In some embodiments, there is provided the method as summarized abovewherein the signal is a TD-SCDMA (time divisional-synchronized codedivision multiple access) signal.

In some embodiments, there is provided the method as summarized abovewherein the signal is a HCR-TDD (High Chip Rate Time Division Duplex)signal.

In some embodiments, there is provided the method as summarized abovewherein the signal is a VHCR TDD (Very High Chip Rate Time DivisionDuplex) signal.

In some embodiments, there is provided the method as summarized abovewherein the signal is a LTE (Long Term Evolution) signal.

In some embodiments, there is provided the method as summarized abovewherein the signal is a GSM (Global System for Mobile Communications)signal.

In some embodiments, there is provided the method as summarized abovewherein performing signal detection and channel estimation upon a signalin respect of a cell comprises using a cell-specific code of that cell.

In some embodiments, there is provided the method as summarized abovewherein the cell-specific code is a midamble.

In some embodiments, there is provided the method as summarized abovewherein the cell-specific code is a reference signal.

In some embodiments, there is provided the method as summarized abovefurther comprising reporting a power measurement based on the channelestimate for cell.

In some embodiments, there is provided the method as summarized abovewherein the power measurement is an RSCP (received signal code power)measurement.

In some embodiments, there is provided a mobile device configured toimplement any one of the methods summarized above.

In some embodiments, there is provided a computer readable medium havingstored thereon instructions for execution by one or more processors of amobile device which when executed perform any one of the methodssummarized above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of channel estimates using a correct midamble and anincorrect midamble;

FIG. 2 is a flowchart of a method of performing channel estimation withselective interference cancellation;

FIG. 3 is a flowchart of another method of performing channel estimationwith selective interference cancellation;

FIG. 4 is a flowchart of a method of performing channel estimation withdetection of erroneous neighbour cell measurements;

FIG. 5 is a flowchart of a method of performing channel estimation withboth selective interference cancellation and detection of erroneousneighbour cell measurements;

FIG. 6 is a block diagram of a mobile device that performs channelestimation with selective interference cancellation;

FIG. 7 is a block diagram of a mobile device that performs channelestimation with detection of erroneous neighbour cell measurements; and

FIG. 8 is a block diagram of a mobile device that performs channelestimation with selective interference cancellation and/or withdetection of erroneous neighbour cell measurements;

FIG. 9 depicts simulation results for unknown interfering cell and nointerference cancellation—static 1 tap channel;

FIG. 10 depicts simulation results for known interfering cell andinterference cancellation—static 1 tap channel;

FIG. 11 depicts simulation results for unknown interfering cell and nointerference cancellation—2 tap equalizer test channel;

FIG. 12 depicts simulation results for known interfering cell andinterference cancellation—2 tap equalizer test channel;

FIG. 13 depicts simulation results for unknown interfering cell and nointerference cancellation—4 tap equalizer test channel;

FIG. 14 depicts simulation results for known interfering cell andinterference cancellation—4 tap equalizer test channel;

FIG. 15 depicts simulation results for unknown interfering cell and nointerference cancellation—4 tap equalizer test channel for detected celland 1 tap for interfering cell;

FIG. 16 depicts simulation results for known interfering cell andinterference cancellation—4 tap equalizer test channel for detected celland 1 tap for interfering cell;

FIG. 17 depicts simulation results for unknown interfering cell and nointerference cancellation—8 tap equalizer test channel;

FIG. 18 depicts simulation results for known interfering cell andinterference cancellation—8 tap equalizer test channel;

FIG. 19 depicts simulation results for AWGN and no interference;

FIG. 20 depicts simulation results for static 8 tap equalizer testchannel and no interference;

FIG. 21 depicts simulation results for fading 8 tap equalizer testchannel and no interference;

FIG. 22 depicts simulation results for Rayleigh fading and nointerference;

FIG. 23 depicts simulation results for various interference cancellationcorner cases;

FIG. 24 depicts simulation results for unknown interfering cell and nointerference cancellation—static 1 tap channel;

FIG. 25 depicts simulation results for known interfering cell andinterference cancellation—static 1 tap channel;

FIG. 26 depicts simulation results for unknown interfering cell and nointerference cancellation—2 tap equalizer test channel;

FIG. 27 depicts simulation results for known interfering cell andinterference cancellation—2 tap equalizer test channel;

FIG. 28 depicts simulation results for unknown interfering cell and nointerference—4 tap equalizer test channel;

FIG. 29 depicts simulation results for known interfering cell andinterference cancellation—4 tap equalizer test channel;

FIG. 30 depicts simulation results for unknown interfering cell and nointerference cancellation—4 tap equalizer test channel for detected celland 1 tap for interfering cell;

FIG. 31 depicts simulation results for known interfering cell andinterference cancellation—4 tap equalizer test channel for detected celland 1 tap for interfering cell;

FIG. 32 depicts simulation results for unknown interfering cell and nointerference cancellation—8 tap equalizer test channel;

FIG. 33 depicts simulation results for known interfering cell andinterference cancellation—8 tap equalizer test channel;

FIG. 34 depicts simulation results for AWGN and no interference;

FIG. 35 depicts simulation results for static 8 tap equalizer testchannel and no interference;

FIG. 36 depicts simulation results for fading 8 tap equalizer testchannel and no interference;

FIG. 37 depicts simulation results for Rayleigh fading and nointerference; and

FIG. 38 depicts simulation results for raw channel estimation outputwith various misalignments of the midamble.

DETAILED DESCRIPTION

In some lab testing for routing area updates, experiments were conductedto examine a case in which the signal of the serving cell is made verysmall from one instant to another and in the same instant another cell'ssignal is made as strong as the previous serving cell's signal has been.For this experiment, both the serving cell and the new cell are assumedto work on the same frequency. Since both cells are on the samefrequency, reselection from a first cell to a second cell on the samefrequency can be referred to as intrafrequency cell reselection. In thissituation, the device under test was not able to find the new cell andstayed with the previous cell whilst assuming that the previous cell isstill in good condition.

Typically, the network supplies a neighbor cell list in respect of aserving cell. This neighbor cell list contains neighbor cells of theserving cell. In the field, there are often situations where due to theway network planning was conducted, the neighbor cell list in respect ofa serving cell supplied by the network lists cells that can be muchweaker than other cells on the same frequencies that are not included inthe neighbor cell list. The result is that the device under test A)reports neighbor cells on these frequencies which are not there at all,or B) reports neighbor cells on these frequencies which are there, butwhich are heavily interfered with by the strong cells which are notincluded in the neighbor cell list on those frequencies, and in so doingreports an incorrect RSCP measurement. The same might be the case for aheavily interfered with serving cell. This may result in failedhandovers and call drops.

In TD-SCDMA, a given cell operates with an assigned frequency orfrequencies from a set of available frequencies and an assignedmidamble. In TD-SCDMA there are 128 basic midamble codes of length 128available for assignment to the different cells. Cells with the samebasic midamble code should not be able to be received on the samefrequency. Appropriate radio network planning should ensure that nomobile device is receiving two cells on the same frequency and the samebasic midamble code. The midamble code is used for channel estimationand differentiation of the cells. The transmitted TD-SCDMA signal has amidamble section during which the midamble is transmitted and a datasection during which other channels are transmitted.

If channel estimation is performed for a cell operating on frequencyF_(i), and (basic) midamble (code) M_(j), but using F_(i) and M_(k) forthe channel estimation (i.e. the right frequency and the wrongmidamble), the result looks like noise. A channel estimate for a cellfor these purposes may be performed by performing correlation of amidamble portion of a received signal (what is assumed to be themidamble portion of the received signal) with the midamble code of thecell. In the absence of interference, this produces a set of taps, eachhaving an amplitude and delay, that collectively represent the impulseresponse of the channel. Note that in some cases, a different cyclicshift of the midamble assigned to a given cell is transmitted to eachmobile device; in this case, the mobile device uses an appropriatechannel estimation window to detect the appropriate cyclic shiftedmidamble and its channel impulse response in the channel estimationwindow.

FIG. 1 shows this result quite clearly. The channel estimate 20 is theresult of the channel estimate derived from a pure received signal ofthe right midamble code. Each channel estimate 30 is the result of thewrong midamble code being used to derive the channel estimate. Forchannel estimate 20, there is one tap for the channel estimate—the othertaps are so small that they are not visible on this graph. For channelestimates 30, these are noisy channel estimates in which the estimatedtaps have on average 1/128 of the power as the overall received signal;the tap position is in units of the CDMA chip duration.

Interference from Cells on Same Frequency:

In the case of the channel estimation for intrafrequency cellreselection (cells operating on the same frequency as that of theserving cell), the interference from the serving cell's midamble andpossibly other data signals is cancelled from the received signals suchthat the channel estimation of the intrafrequency neighbor cells is donewith the resultant signal.

More generally, in the case of channel estimation for a second celloperating on a frequency, where there is a first cell (or cells)operating on the same frequency as that of the first cell, theinterference due to the first cell's midamble and possibly due to otherchannel(s) transmitted from the first cell in the data section iscancelled from the received signals such that channel estimation of thesecond cell is done with the resulting signal. In the case, the firstcell may be another known cell that is not necessarily the serving cell.

The following is an example of the steps which might be performed withthe midamble portion of the to be detected signal and the midambleportion of a known to be cancelled signal:

1. Channel estimation of the first received signal with the midamble ofthe known to be cancelled cell.

2. Post processing of the channel estimate by identifying tapscontaining only noise (e.g. lower than a threshold with respect to theoverall RX power). These taps are removed from the channel estimate bysetting them to zero.

3. Reconstructing the midamble and possibly the rest of the data signalof the known cell with the postprocessed channel estimate. In case themidamble of another cell is received, it usually has a different timingthan that of the to be detected cell. This means that it could well bethat a data section of a burst of the to be cancelled cell isoverlapping with the midamble of the to be detected cell. In someembodiments, in order to benefit further from interference cancellation,the part of the data section overlapping with the to be detectedmidamble is cancelled as well. The result is subtracted from the firstreceived signal to get a second received signal. This is quite effectivein the case the known to be cancelled cell has almost the same or a lothigher power than the other cells.4. The to be detected cells are then detected by means of processing thesecond received signal with their midamble codes.

There may be a case where there is almost only noise/interference. Thiswill be the case where the component from the known to be cancelled cellis very weak compared to the overall signal, or absent altogether. Inthis case, the channel estimate of the known cell looks like noise. Inthis case, it will not be possible to distinguish a useful channelimpulse response in the channel estimate because the noise is then alsocreating very strong taps. Typically, the channel impulse response isviewed as containing only a few taps each with a small power.

A simple algorithm then will see only the random noisy channel impulseresponse taps and is not able to decide whether the channel estimate isuseful or not. It will assume that it has a channel impulse response(CIR) with strong multiple path propagation and will only chose the veryweak taps to be removed. Most of the taps will pass the post processing.

Note that at least one tap is to be removed because otherwise theinterference cancelled midamble will be the same as the receivedmidamble and the result after interference cancellation will be zero inthe midamble section of the received signal.

The result is that the known cell has still a quite a large apparentnoise power (for the RSCP measurement a channel estimation window of 16of the 128 taps is taken—9 dB below the received power of the firstreceived signal if all 16 taps are considered). If this cell is thentaken for interference cancellation the result will be that, thereconstructed signal is almost the same as the first received signal andthe intrafrequency cell cannot be detected from the remaining debris ofthe received signal. In case an algorithm is not detecting thissituation, a small result will be measured and reported even if the tobe detected cell comes in very strong. In case the known cell is subjectto measurement, a RSCP measurement result of about 9 dB below theinterference plus noise would be reported—even if the cell is not thereany more.

Solutions to Measurement in the Presence of Cells on the Same Frequency

Selectively Perform Interference Cancellation

In a first embodiment, in case a known cell (such as a serving cell oranother cell operating on the same frequency as the to be detected cell)could be interference cancelled from a first receive signal,interference cancellation is switched off if the known cell has areceive power (for example as determined by the sum of tap powers afterthe channel estimation) of less than or equal to a defined fraction(e.g. a defined fraction IC_threshold) of the overall receive power ofthe first receive signal, or is at least a defined amount less than theoverall receive power. In other words, interference cancellation is onlyperformed for a known cell whose power is large enough relative to thepower of the first receive signal as defined by a threshold. Throughoutthis description, it is to be understood that a threshold can be afactor, for example a fractional factor, e.g. 20%, or a defined amount(in dB or other units).

Referring now to FIG. 2, a flowchart of a method provided by anembodiment of the application will now be described. This method may,for example, be performed by a mobile device. The method begins in block2-1 with processing a signal to produce an interference cancellationcomponent that is an estimate of a component of the signal that is dueto a first cell. The signal may, for example, be a received TD-SCDMAsignal, but may alternatively be a signal from another standard,examples of which were provided in the background. The component due tothe first cell may be a component from a serving cell, or from anotherknown cell. The method continues in block 2-2 with performing signaldetection (e.g. channel estimation) of a to be detected cell on the samefrequency as that of the first cell based on the signal minus theinterference cancellation component if the interference cancellationcomponent has a power that is large enough compared to the total powerof the signal as defined by a first threshold. In block 2-3, if theinterference cancellation component has a power that is not large enoughcompared to the total power of the signal as defined by the firstthreshold, signal detection of the to be detected cell is performedbased on the signal without having subtracted the interferencecomponent.

In some embodiments, processing the signal to produce the interferencecancellation component of a first cell that is the to be cancelled cellinvolves the following steps:

generating a channel estimate for the first signal using a cell-specificcode of a first cell;

post processing of the channel estimate by identifying certain taps andremoving these taps from the channel estimate;

using the cell-specific code (and possibly part of the rest of the datasignal) with the postprocessed channel estimate to produce theinterference cancellation component.

The cell-specific code may, for example, be the previously referencedmidamble code, but other cell-specific codes may alternatively beemployed. The specific form of the cell-specific code may differdepending upon the radio access technology and the part of the signal tobe cancelled.

In some embodiments, identifying certain taps to be removed involvesidentifying taps that each have a power that is smaller than the totalpower of the first signal as defined by a second threshold.

Perform Channel Estimation with and without Interference Cancellation

In a second embodiment, channel estimation for a to be detected cell inthe presence of a known cell operating on the same frequency isperformed both with and without the interference cancellation step, anda decision is made on which channel estimate to use. The following is aspecific example of how this decision might be made. A channel estimateof the known cell is generated, and post processing is performed toremove certain taps and an interference component is generated based onthe post processed channel estimate for the known cell. A second signalis reconstructed by removing the interference component. Then thechannel estimation for the to be detected cell is performed based on thesecond signal, and post processing is again performed to remove certaintaps to produce a first post processed channel estimate for the to bedetected cell. In addition, channel estimation for the to be detectedcell is performed using the first signal without removing theinterference component, and this is post processed by removing certaintaps to produce a second post processed channel estimate. The sum of thepowers of taps remaining after the postprocessing of the to be detectedcell is determined for both the first post processed channel estimateand the second post processed channel estimate. For each post processedchannel estimate, the ratio of the sum of powers of the remaining tapsto the average/median power of the taps for the corresponding channelestimate with no post processing is determined. Then the channelestimate with the larger power ratio is selected. Other methods ofselecting between the two results may alternatively be employed.

Referring now to FIG. 3, shown is a flowchart of a method provided by anembodiment of the application. The method begins with processing asignal to produce a channel estimate in respect of a first cell in block3-1. In block 3-2, the channel estimate is post-processed by identifyingcertain taps and removing these taps from the channel estimate toproduce a post-processed channel estimate. In block 3-3, first signaldetection of a to be detected cell is performed based on the signalminus an interference cancellation component of the first cell, andsecond signal detection of the to be detected cell is performed based onthe signal without having subtracted the interference cancellationcomponent of the first cell. In block 3-4, a selection is made betweenthe first signal detection and the second signal detection, for example,using the method described above. In case there is no suitable knowncell to perform interference cancellation with, signal detection iscarried out without interference cancellation.

Discard Unreliable Results

In a third embodiment, a decision is made as to whether the measurementresult for a cell (the to be cancelled cell and/or the to be detectedcell) has become unreliable. For example, if the combined power of thechannel impulse response taps surviving the postprocessing (either thepost processing that is performed to produce the interferencecancellation component, or the post processing that is performed toproduce the channel estimate for the to be detected cell, or both) issmall enough compared to the overall receive power of the to beprocessed signal (first receive signal) as defined by a threshold, themeasurement result may be considered unreliable. Here the threshold maydiffer depending on whether interference cancellation is being performedin the process to detect this cell or not. In the event the mobiledevice is required to report on the particular neighbour cell, themobile device may report the lowest reportable receive power instead. Incase the mobile device is required to process the result—in anotheralgorithm for example—it can perform the processing using a very smallpower. Alternatively, the measurement result can be discardedaltogether. In the event the mobile device is not required to report onthe particular cell, the result may be either not be reported, or themobile device can choose whether or not to report. This decision may bemade on a context specific basis. For example, if reporting a singlehost cell on a frequency, there is the possibility of being handed overto that cell, and as such, the mobile device may choose not to report onthat cell to prevent such a handover. On the other hand, if there is areport on a group of ghost cells having a similar power, this may havethe effect of preventing the network from handing over.

The following is a specific example of a method based on the aboveembodiment using interference cancellation. Begin by processing a signalto produce an interference cancellation component that is an estimate ofa component of the signal that is due to a first cell by:

generating a channel estimate for the signal using a cell-specific codeof a first cell;

post processing of the channel estimate by identifying certain taps andremoving these taps from the channel estimate to produce a postprocessed channel estimate for the first cell;

producing the interference cancellation component using the postprocessed channel estimate.

Perform channel estimation of a to be detected cell operating on thesame frequency as the first cell based on the signal minus theinterference cancellation component by:

generating a channel estimate for the to be detected cell using acell-specific code of that to be detected cell;

post processing of the channel estimate for the to be detected cell byidentifying certain taps and removing these taps from the channelestimate to produce a post processed channel estimate for the to bedetected cell;

The method further comprises at least one of:

a) discarding the post processed channel estimate for the to be detectedcell and/or reporting a lowest reportable value/other very small valueand/or not reporting the channel estimate if a combined power of thetaps of the post processed channel estimate for the to be detected cellis small enough compared to a total receive power for the signal by athreshold amount; and

b) discarding the post processed channel estimate for the to be detectedcell and/or reporting a lowest reportable value/other very small valueand/or not reporting the channel estimate if a combined power of thetaps of the post processed channel estimate for the first cell is smallenough compared to a total receive power for the signal by anotherthreshold amount. In the case of a) the post processed channel estimatefor the to be detected cell is considered unreliable. In the case of b)the post processed channel estimate for the to be cancelled cell isunreliable which leads to the post processed channel estimate for the tobe detected cell to also be considered unreliable, since interferencecancellation based on an unreliable channel estimate was performed.

Here optionally for the measurement of a specific power such as the RSCPof the P-CCPCH the combination of power might be restricted to thechannel estimation window of the P-CCPCH power. In later describedembodiments the power of the channel estimates for other physicalchannels used together with the P-CCPCH on the same time slot will betaken into the equation as well.

In some cases, only a) is implemented, or only b) is implemented. Insome cases, both a) and b) are implemented. In case of b) anotherestimate for the to be detected cell without interference cancellationmight be generated or another cell to perform interference cancellationwith may be found.

In some embodiments, a combination of one or more of the first, second,and third embodiments described above are implemented.

FIG. 4 is a flowchart of another example. In block 4-1, channelestimation is performed of the first received signal with the midambleof a to be detected neighbour cell. In block 4-2, the channel estimateis post processed by identifying certain taps (e.g. taps containing onlynoise, for example as determined by being small enough with respect tothe overall receive power as defined by a threshold). These taps areremoved from the channel estimate, for example by setting them to zero,to produce a post-processed channel estimate. The combined power of theremaining taps of the post-processed channel estimate is determined inblock 4-3 and the overall power of the first signal is determined inblock 4-4. If this combined power is small enough compared to theoverall power of the first signal as defined by a threshold, then theresults are discarded and/or a minimum value is reported, and/or notreporting the channel estimate (block 4-5).

A cell that has an unreliable measurement result, for example asdetermined by the above-summarized method, is also referred to herein asa “ghost cell”. If the combined power of the remaining taps is smallenough compared to the overall power of the signal as defined by athreshold, also referred to as a ghost detection threshold, then theconclusion is reached that the cell is a ghost cell. The ghost detectionthreshold may be different for cases where interference cancellation hasbeen employed (ghost detection threshold with IC) as opposed to caseswhere interference cancellation has not been employed (ghost detectionthreshold without IC).

Note that this approach may also be applied to signal detection of a tobe detected cell in the presence of interference from a known cell(serving cell or other cell) operating on the same frequency todetermine whether the known cell is significantly present in the overallreceived signal; then interference cancellation is employed for theknown cell or not based on this determination as described previously.In other words, the detection of a cell as being a ghost cell or not isused as a trigger for the decision to perform interference cancellationor not. The approach can be used to decide whether the measurementreport for a to be detected cell, or a to be cancelled known cell or ato be cancelled unknown cell is unreliable.

FIG. 5 is a flowchart of a specific example of a method that combinessome of the methods described above.

Interference cancellation may or may not be performed. If it is, thenyes path block 5-1 is followed. If it is not, then no path, block 5-1 isfollowed. Assuming interference cancellation is performed, then in block5-2, channel estimation of the to be cancelled signal is performed. Inblock 5-3, the channel estimate is post processed, for example byremoving certain taps. The outcome of block 5-3 is the channel impulseresponse. In block 5-4, an assessment of whether the post processedchannel estimate for the to be cancelled cell is reliable or not ismade. If the post processed channel estimate is unreliable, then block5-4, no path is followed which results in the method continuing at block5-8 without subtracting out a component due to the to be cancelled cell.If the post processed channel estimate is reliable, then the methodcontinues at block 5-6 with reconstructing a signal of the to becancelled cell using the post processed channel estimate.

Interference cancellation for additional cells may be performed in whichcase no path 5-7 is followed. Otherwise, channel estimation is performedfor the to be detected signal at block 5-8. This can be post processedby removing certain taps that are considered to be noise. In blocks 5-9,5-10, a decision is made as to whether the channel estimate for the tobe detected cell is reliable or not based on whether or not “condition1” is fulfilled, as detailed below. If it is reliable (no path block5-10), then the RSCP is calculated normally and reported. If it is notreliable (yes path, block 5-10), then the RSCP is set to a lowestreportable value, or a very small value, or not reported at all.

In FIG. 5, block 5-4, the post processed channel estimate for the to becancelled cell is determined to be reliable if the power of the taps ofthe post processed channel estimate is large enough compared to theoverall power of the receive signal as defined by a threshold. In thespecific example depicted, if the power of the received signal (the sumof the taps output in block 5-2) over the power of the taps of the postprocessed channel estimate (the sum of the taps output in block 5-3(channel impulse response)), is less than an IC_threshold, then the postprocessed channel estimate is considered reliable.

Similarly, in block 5-10, the channel estimate for the to be detectedcell is determined to be reliable if the power of the taps of thechannel estimate is large enough compared to the overall power of thereceive signal as defined by a threshold. Different thresholds may beapplied depending on whether interference cancellation was employed ornot. For example:

If interference cancellation (IC) has been applied, condition 1 isfulfilled and the result considered unreliable if:RSCP<(Ghost detection threshold with IC applied)*overall receive powerof received signal) and receive power of receive signal>(lower ghostdetection bound)*thermal_noise_power

If IC has not been applied, condition 1 is fulfilled and the resultconsidered unreliable if:RSCP<(Ghost detection threshold without IC applied)*overall receivepower of receive signal and receive power of receive signal>(lower ghostdetection bound)*thermal_noise_power.

In the above, the RSCP is the sum of the taps remaining for the firstmidamble shift/first channel estimation window of interest in time slotzero. More generally, it can simply be the sum of a set of taps ofinterest. In some embodiments, the ghost detection thresholds are onlyapplied if the receive power is more than an amount “lower ghostdetection bound” (for example 3 dB) greater than the thermal noise. Thisapproach has been taken in the example of FIG. 5.

Simulation Results

Simulation results were obtained. In the following, the performance ofthe algorithm has been simulated with the following further improvementson the measurement procedure:

First improvement: When post processing a channel estimate (either for ato be detected cell or a to be cancelled cell), instead of deleting allthe taps for the channel estimate which are less than a certainthreshold below the overall received power the following algorithm wasapplied:

-   -   a) Set the result vector to zero    -   b) Calculate the average power of all the channel impulse        response vector    -   c) Find the strongest channel impulse response tap    -   d) If the strongest channel impulse response tap is stronger        large enough as defined by a threshold compared to the average        power of the remaining channel impulse response taps take this        tap over to the result vector, remove this tap from the channel        impulse response vector and calculate the average power of the        remaining channel impulse response taps in the channel impulse        response vector. Go to step c)    -   e) If the condition of d) is not fulfilled terminate the post        processing and thus discard all the remaining channel impulse        response taps.

The result is that each of the taps other than the certain taps has apower that is large enough compared to the average power of taps otherthan the certain taps as defined by the threshold.

Second Improvement: The average power of all the taps being discarded inthe first improvement described above is determined. The power of allthe remaining channel impulse response taps is adjusted downward by thataverage for the purpose of generating RSCP measurements.

On the discarded taps there is the assumption that only noise is onthem. Since usually more than one tap is discarded the result can beaveraged.

On the taps surviving the post processing (i.e. the ones that are notdiscarded) there is both noise and signal on them. When determining theRSCP measurement, the noise power can be removed from the surviving tapsby subtracting the average noise power from each of the surviving tapspower to produce noise-adjusted surviving taps and the RSCP is thendetermined by summing the noise-adjusted surviving taps.

Definitions and Parameters for Simulations

Definition of Ghost Cell:

For these simulations, a ghost cell is a cell with an unreliable RSCPresult. A more general definition of a ghost cell was presented above.Once all the channel impulse response taps have been discarded (set to0) this is considered a ghost cell as well. The RSCP is measured on the16 taps of midamble shift 1 in TS0.

The standard channel estimation algorithm and a simple interferencecancellation algorithm has been chosen to simulate the performance ofthis algorithm. The parameters of this algorithm have been optimized forthe purpose of RSCP measurements. For the purpose of detection processesanother set of parameters has to be chosen.

The chosen parameters are the following:

Detected Midamble

The to be detected midamble has the code 48.

Interfering Midamble

The interfering midamble has the code 49.

Detection Threshold for the to be Detected Midamble:

This is the threshold for the first improvement described above for thechannel estimation of the cells to be detected. It is set to 8 dB.

Detection Threshold for to be Cancelled Midamble:

This is the threshold for the first improvement described above for thechannel estimation of the cells to be cancelled. It is set to 10 dB.Since IC only makes sense once the cells to be cancelled are very strongit is set to a tougher value than for the to be detected midamble.

IC Threshold

Interference cancelling is not used once the interfering cell is not inthe neighbor cell list/or it is not the serving cell. In case a knowncell could be cancelled, IC is switched off if the to be cancelled cellhas an RX power (sum of tap powers after the channel estimation) of lessthan or equal to 3 dB of the overall RX power.

Ghost Detection Threshold without IC Applied

For the case that no IC is applied the cells with a RSCP that is anamount “ghost detection threshold without IC applied” (for example 10 dBor more in the simulations) less than the RX power of the RXed midambleare considered as ghost cells.

Ghost Detection Threshold with IC Applied

For the case that IC is applied the cells with a RSCP that is an amount“ghost detection threshold with IC applied” (for example 35 dB or morein the simulations) less than the RX power of the RXed midamble areconsidered as ghost cells. This threshold is depending on the AD wordwidth and the processing word width.

Lower Ghost Detection Bound

In some embodiments, the ghost detection thresholds are only applied ifthe RX power is large enough compared to thermal noise as defined by anamount “lower ghost detection bound” (for example 3 dB in thesimulations). In the simulations, the thermal noise has an RX power of 0dB.

Not Simulated Brute Force Algorithms:

-   -   1. In case a ghost cell is detected with the ghost detection        threshold and no IC is used the mobile device could        systematically search for the cells and their midamble codes        which are not in the neighbor cell list and in case they are        strong enough apply IC with them. This approach would find the        interferer which is unknown to the receiver once the ghost cell        is detected.    -   2. In case a ghost cell is detected with the ghost detection        threshold it is only assumed to be a ghost cell once its BCCH        cannot be detected successfully. That means in case the BCCH can        be ready the cell is there with sufficient strength to be        detected and is thus no ghost cell.    -   3. In some embodiments, once the algorithm looks systematically        for cells not being in the neighbor cell list (e.g. in case        there is a ghost cell) and finds none then either the ghost cell        is finally detected or a detection of the BCCH is attempted to        determine whether it is a ghost cell.        Equations        These are the Equations for a Simple Channel Estimation        Algorithm for TD-SCDMA: (Other Standards HCR_TDD and VHCR-TDD        have Different Parameters)        unfiltered=ifft(fft(input))./fft(to_be_detected_midamble)  (1)

Where ./ means the division of the vector elements and input andto_be_detected_midamble are vectors of length 128. fft and ifft are theFourier Transformation and the inverse Fourier Transformationrespectively.

After that, postprocessing is performed by creating a vector resultwhich is the vector unfiltered with all the elements:

$\begin{matrix}{{result}_{i} = \{ {\begin{matrix}{unfiltered}_{i} & {{{if}\mspace{14mu}\frac{{{unfiltered}_{i}}^{2}}{interfer\_ P}}>={detection\_ threshold}} \\0 & {else}\end{matrix}\mspace{20mu}{where}} } & (2) \\{\mspace{79mu}{{interfer\_ P} = {\frac{\sum\limits_{{result}_{i} = 0}{{unfiltered}_{i}}^{2}}{\sum\limits_{{result}_{i} = 0}1}.}}} & (3)\end{matrix}$(3) is the interference power of all the CIR taps being cancelled in thepreprocessing process.

The solution to (2) and (3) is first sorting the unfiltered channelimpulse response according to the absolute value, setting result to bethe zero vector, then calculating the interfere_P assuming that allelements of the vector result are 0, then applying the condition of thestrongest unfiltered element.

If the equation (2) is fulfilled making the strongest unfiltered elementpart of the result vector—if not setting the corresponding element andall the remaining elements of the result vector to zero.

Then update interfere_P (3) and then working with the second strongestelement in the vector unfiltered and so on until the condition is notfulfilled any more.

detection_threshold is either the “Detection threshold for the to bedetected midamble” of the “Detection threshold for the to be cancelledmidamble” depending on whether IC is used or not.

Note there are different result vectors for the to be cancelled midambleand the to be detected midamble.

These Equations Apply for the Interference Cancellation Algorithm:

result(midamble_to_be_cancelled_signal) is the result if the channelestimation with the detection_threshold set to the “detection thresholdwith IC”

$\begin{matrix}{{total\_ power} = \frac{\sum\limits_{i}{( {1{st}\mspace{14mu}{RX}\mspace{14mu}{signal}} )_{i}}^{2}}{128}} & (4)\end{matrix}$is the total RX power of the 1^(st) RX signal in the midamble portion ofthe to be cancelled cell. In case the timing of the to be detected celland the to be cancelled cell is different a different total_power has tobe used for both and their respective midamble portions have to betaken.And

$\begin{matrix}{{{power\_ to}{\_ cancel}} = {\sum\limits_{i}{( {{result}( {{midamble\_ to}{\_ be}{\_ cancelled}{\_ signal}} )} )_{i}}^{2}}} & (5)\end{matrix}$

The power of the to be cancelled signal.

If(total_power/power_to_cancel)>IC_threshold  (6)is fulfilled then create the 2^(nd) RX signal with:(2^(nd) RX signal)=(1^(st) RXsignal)−conv(reconstructed_signal_to_be_cancelled,result(midamble_to_be_cancelled_signal))  (7)

The “reconstructed signal to be cancelled” is calculated by detectingthe burst of the interfering cell and recreating the transmitted signalof the transmitting node B. Here shortcuts might be taken e.g. by onlyreconstruction (parts of) the signal overlapping with the midamble to bedetected. conv(x,y) is the convolution of the two vectors x and y.

If (6) is not fulfilled (8) is applied.(2^(nd) RX signal)=(1^(st) RX signal)  (8)

Then the 2^(nd) RX signal is the input signal of the channel estimationalgorithm described above.

These Equations Apply for the RSCP Measurement:

The vector of the channel impulse response power is created bymanipulating the individual elements of the result like this:CIR_power_(i)=|result_(i)|²−interfer_P  (9)

With result and interfer_P being the solutions to (2) and (3) for thestep creating the CIR of the to be measured cell.

Then the RSCP is

$\begin{matrix}{{RSCP} = \{ \begin{matrix}{\sum\limits_{i = 1}^{16}{result}_{i}} & \begin{matrix}\; & \begin{matrix}{{if}\mspace{14mu}( {\sum\limits_{i = 1}^{16}\;{result}_{i}} )*} \\{( {{ghost}\mspace{14mu}{detection}\mspace{14mu}{threshold}} )>={total\_ power}} \\{{{or}\mspace{14mu}{total\_ power}}<=} \\( {{lower}\mspace{14mu}{ghost}\mspace{14mu}{detection}\mspace{14mu}{bound}} )\end{matrix}\end{matrix} \\0 & {{else}\mspace{405mu}}\end{matrix} } & (10)\end{matrix}$where the ghost detection threshold is either the one with IC or the onewithout IC depending whether IC has been applied before.4 Simulations and their DiscussionThere are two kinds of simulations performed:

-   -   1. Simulations with interfered to be measured cells. There the        RX power of the to be measured cell should be 55 dB. The        simulations are run versus the ISR (Interference to Signal        Ratio). An ISR of 10 dB means that the interfering cell is 10 dB        stronger than the to be measured cell. In these simulations for        each point the phase offsets 1-360 degree in-between interfering        cell and measured cell have been taken. In case of fading        channels the RX powers have been normalized such that        instantaneous ISRs are measured. Otherwise the fading statistics        would overlay the simulation results.    -   2. Simulations without interfering cells. Here the effect of the        thresholds and measurement procedures on normal measurements is        studied. The simulations are run versus the SNR. The noise power        is held at a level of 0 dB. The SNR is the average SNR in case        of a fading channel being used. 400 independent snapshots per        point have been made.        For each campaign 3 results are obtained:    -   1. The measured RX power in dB versus SNR or ISR    -   2. The standard deviation of the measured RX power in dB versus        SNR or ISR    -   3. The percentage of the ghost cells being detected versus SNR        or ISR        Not Simulated Improvements:

Once the detection threshold for the to be detected midamble is set to 6dB the results without interference look better: less bias at low SNRsand less ghost cells detected.

If this behavior is desired, change the detection threshold for the tobe detected midamble from 8 dB to 6 dB once the total TX power fallsbelow the Lower ghost detection bound.

FIG. 9 depicts simulation results where there is an unknown interferingcell and no interference cancellation, and a static 1 tap channel isassumed. Here the ghost cell detection algorithm almost works perfectly.At 10 dB ISR the algorithm cuts off. At that point the measured RX poweris not significantly higher than it should be.

FIG. 10 depicts simulations results where there is a known interferingcell and interference cancellation, and a static 1 tap channel isassumed. The algorithm works perfect: Cut off at 35 dB and almost nostandard deviation once the interference cancellation is switched off (0dB ISR).

FIG. 11 depicts simulation results where there is an unknown interferingcell and no interference cancellation, and a 2 tap equalizer testchannel is assumed. The 2 tap equalizer test channel applies for bothdetected cell and interfering cell. It can be seen that the zone wherethe ghost cells are beginning to be detected is widening up. This is tobe expected since for cases where both detected taps have about the sameRX power their individual RX power is only half as big as for the 1 tapcase. It is noted that the RX power and the standard deviation is onlytaking the cells into account which are detected as no ghost cells.

FIG. 12 depicts simulation results where there is a known interferingcell and interference cancellation, and a 2 tap equalizer test channelis assumed. The algorithms works still perfect but the standarddeviation is greater than for the 1 tap channel.

FIG. 13 depicts simulation results where there is an unknown interferingcell and no interference cancellation, and a 4 tap equalizer testchannel is assumed. The 4 tap equalizer test channel applies for bothdetected cell and interfering cell. It can be seen that the zone wherethe ghost cells are beginning to be detected is widening up more. Thisis to be expected since for cases where both detected taps have aboutthe same receive power their individual RX power is only a quarter asbig as for the 1 tap case. It is noted that the RX power and thestandard deviation is only taking the cells into account which aredetected as no ghost cells. It is also noted that a 4 tap equalizer testchannel is quite unrealistic already. In any case even though 1 or 2% ofall ghost cells are not detected this seems to be indicate the limit ofthis algorithm.

FIG. 14 depicts simulation results where there is a known interferingcell and interference cancellation, and a 4 tap equalizer test channelis assumed. It can be seen that there is a greater standard deviation,but that in general, the approach is still working fine.

FIG. 15 depicts simulation results where there is an unknown interferingcell and no interference cancellation, and a 4 tap equalizer testchannel for detected cell and 1 tap for interfering cell area assumed.Once the more realistic scenario is simulated that the interferer hasjust 1 tap the result is looking fine again.

FIG. 16 depicts simulation results where there is a known interferingcell and interference cancellation, and a 4 tap equalizer test channelfor detected cell and 1 tap for interfering cell are assumed. Noproblem.

FIG. 17 depicts simulation results where there is an unknown interferingcell and no interference cancellation, and an 8 tap equalizer testchannel is assumed. The 8 tap equalizer test channel applies for bothdetected cell and interfering cell. It can be seen that the zone wherethe ghost cells are beginning to be detected is widening up most. Thisis to be expected since for cases where both detected taps have aboutthe same RX power, their individual RX power is only an eighth as big asfor the 1 tap case. It is noted that the RX power and the standarddeviation is only taking the cells into account which are detected as noghost cells. It is noted that an 8 tap equalizer test channel isunrealistic. In any case even though 1 or 2% of all ghost cells are notdetected this seems to indicate the limit of this algorithm.

FIG. 18 depicts simulation results where there is a known interferingcell and interference cancellation, and an 8 tap equalizer test channelis assumed.

FIG. 19 depicts simulation results for AWGN and no Interference. Withnegative SNR the algorithm becomes impaired by noise. With too low SNRghost cells are detected because no useful channel impulse response tapcan be detected inside the channel estimation window. The result isbecoming biased below −10 dB SNR. That would be the case if nomodifications would have been applied anyway.

FIG. 20 depicts simulation results for a static 8 tap equalizer testchannel and no interference. With negative SNR a ditch can be seen with8 taps having the same power each tap is having only ⅛^(th) of the poweras in AWGN. Some taps are overshadowed by noise.

FIG. 21 depicts simulation results where there is a fading 8 tapequalizer test channel and no interference. Due to the fading nature ofthe channel more ghost cells are detected once the fading lets the celldrop below the noise level. This is normal.

FIG. 22 depicts simulation results where there is Rayleigh fading and nointerference. Due to the strongest fading nature of the channel evenmore ghost cells are detected once the fading lets the cell drop belowthe noise level. This is normal.

FIG. 23 depicts simulation results for various interference cornercases. Based on observations of the logs from the field trials, it wasobserved that the IC algorithm might be responsible for some call dropsas well. In this case the serving cell is impaired by a strong intracellinterferer not being in the neighbor cell list. The radio link isestablished in fine conditions. After a while the interferer isfalsifying the measurement results. The serving cell is measured with ahigh power and the IC reduces the RX power of the intra-neighbor cellsin the neighbor cell list to a low power. Since the call is not on TS0this situation stays undetected until the interfering cell is opening acall in the used DL TS. Then the interference in the traffic TS iscausing RLF.

It has been tried to model this situation with the following additionalsettings:

-   -   1. Adding thermal noise at 0 dB power.    -   2. Reducing the RX power of the to be detected cell to 25 dB        above the thermal noise power.    -   3. Using static channels        We show only the RX power versus the ISR for the 4 sets of        parameters:        Case 1: parameters as suggested before.        Case 2: following parameters changed:    -   Detection threshold for to be cancelled midamble 7 dB (letting        more taps to be considered for the IC process)    -   IC threshold 30 dB (IC is not disabled based on the total RX        power)        Case 3: following parameters changed:    -   Detection threshold for to be cancelled midamble 6 dB (letting        more taps to be considered for the IC process)    -   IC threshold 30 dB (IC is not disabled based on the total RX        power)        Case 4: following parameters changed:    -   Detection threshold for to be cancelled midamble 5 dB (letting        more taps to be considered for the IC process)    -   IC threshold 30 dB (IC is not disabled based on the total RX        power)

From the results it can be seen that case 1 is fine. Case 2 is showingsigns of deterioration.

Cases 3 and 4 show a significant power reduction of the very strong tobe detected cell. The same is seen for the RSCPs of the intra neighborcells in the traces. The assumption is that the current IC algorithm ishaving a detection threshold which is variable: High for high SNR in theCIR and low for low SNR in the CIR.

Additional Interference from Other Signals

In some embodiments, the signal detection is performed on the basis of asequence that is transmitted by the base station at the same time thatother signals are transmitted by the base station, with codechannelization being used to distinguish between the various signals.The other signals are transmitted with different channelization codesbut with the same sequence (e.g. midamble) using a different cyclicshift. This enables the related interference of these codes to beinvisible for the to be measured channel impulse response (e.g. the onesused for the P-CCPCH).

Recall a ghost cell is a cell with an unreliable measurement result. Thefollowing is another example of a ghost cell detection criterion.

Ghost Detection Threshold:

-   -   a cell with a power of a to be detected power of the P-CCPCH        channel impulse response (RSCP) (more generally the power of a        sequence for signal detection, for example in the first channel        estimation window) within a received signal that is smaller than        the total receive power (or the total corrected receive power        where correction is applied) of the received signal during the        midamble period as defined by a threshold (more generally during        the period of the sequence for signal detection) is considered        to be a ghost cell.

For example, in TD-SCDMA networks, the sequence for signal detection isthe midamble, and the midamble for signal detection purposes is betransmitted on the P-CCPCH on time slot 0 (TS0) using the first twochannelization codes, while at the same time (i.e. during TS0) one ormore of S-CCPCH, PICH and FPACH are also transmitted using respectivechannelization codes using a different midamble shift than the P-CCPCH.These channels are usually allocated with a default midamble allocationand K=4, where “K=4” means that 4 midamble shifts are employed, andthere is a default mapping between the midamble shifts and thechannelization codes. Other mappings may alternatively be employed. Thecontent of TS0 can be specified in system information, for example inSIB (system information block) 5.

Once a mobile station detects this to be the case for one cell, it canalso be assumed to be the case for the other cells in the same area. Itis the nature of these channels that they do not appear all the time.Some (FPACH (e.g. 1 code) and S-CCPCH (e.g. 3 codes) for DCCH/DTCH)might be beamformed but PICH and S-CCPCH for CCCH are transmitted in anomni cell fashion. In the best case the P-CCPCH has TS0 on its own andin the worst case, assuming a scaling of the powers of the midambleshifts according to the number of used codes there is 3 times more poweron the midamble of TS0 (and not only there). The power of a midambleshift is the same as the power of the codes used with it (multiple codesmight use the same midamble shift). It is assumed that the power of thecodes is all the same to reach the cell edge with all codes. There are 2codes for the P-CCPCH, e.g. 3 codes for the S-CCPCH, e.g. 1 code for theFPACH, for a total of 6 codes altogether—three times as many codes as ifthere were only 2 for the P-CCPCH. Since then there are 3 times morecodes there is 3 times more power than in the best case where there isthe P-CCPCH alone on TS0.

Additional Interference Cancellation of Components Due to a to beCancelled Cell:

In a case the other channels are used in a known or an unknown neighbourcell, there will be more interference from the known and unknownneighbor cells. This is up to e.g. 5 dB (3 times) more. In thesimulations below, the ISR (Interference to Signal Ratio) reflects theratio of the P-CCPCH powers only. This reflects that normally the cellsare transmitting the P-CCPCH alone. For the worst case the graphs willbe shifted by 5 dB or more towards lower ratios. The additionalinterference will have the same properties as for the P-CCPCH case apartfrom the increase in power.

In some embodiments, for the interference cancellation algorithms, theadditional paths (also referred to as taps) for the additionally usedmidamble shifts of a to be cancelled cell are cancelled from thereceived signal as well. This is done as described before (Estimation ofthe interferer, identification of the noise in the estimate, thenreconstruction the receive signal of the to be cancelled cell andsubtracting it from the receive signal). However, with each of the pathsbeing cancelled also a fraction of the power of the to be detectedmidamble is taken away. It has to be expected that the estimation of theto be detected midamble power will be more biased than before.

Removal of Contributions to Total Receive Power Due to Other Channels ofto be Detected Cell:

The total receive power may, for example, initially be calculated as thesum of the powers of the samples divided by the number of samples. Forthe case of interference cancellation, this is done before interferencecancellation.

There are other midamble shifts that may simultaneously be used in theto be detected cell. Since the channel estimation algorithm is unbiased,the P-CCPCH channel impulse response can be detected as before. Howeveronce other midamble shifts are used, their power will also contribute tothe overall receive power and thus without any other change more ghostcells are detected, since the ghost cell detection is a function of thepower of the to be detected cell compared to the overall received power.Keep in mind here there the ghost detection threshold is differentdepending on whether interference cancellation is performed or not.

In some embodiments, the other midamble shifts of the to be detectedcell are considered for the ghost cell detection by performingsubtraction of the power of the other midamble shifts (the ones notbelonging to the P-CCPCH) from the total receive power before applyingthe ghost detection threshold.

An assumption is made that the other channel estimation windowsassociated with other midamble shifts (for example 4 channel estimationwindows associated with K=4 shifts) could contain used midamble shifts.If this approach is taken, there are three potential errors that mayresult:

-   -   a. There are midamble shifts detected which are not there.        Subtracting an amount from the received power associated with a        midamble shift that is not there will reduce the received power,        and make it easier for a cell to pass the ghost cell detection        threshold. This will in tendency prevent ghost cells from being        detected.    -   b. There are midamble shifts not detected which are there. This        will in tendency result in ghost cells being detected which        should not be detected. In this case, an amount of received        power associated with a midamble shift is not subtracted because        it was not detected; the receive power will not be reduced to        the extent it should have been, and it will be harder for a cell        to pass the ghost cell detection threshold.    -   c. Detected midamble shifts are detected with erroneous path        positions and path powers.

In short this will result in a bigger transition zone (zone ofuncertainty) from no ghost cells being detected to ghost cells beingdetected. This is because the measurement errors may provoke a detectionof a ghost cell which is not there. Then the measured power is onlybelow the threshold by chance. Similarly, the measurement errors provokethat a ghost cell is not detected. Then the measured power is above thethreshold by chance.

In some embodiments, the following changes are implemented compared tothe previously described embodiments, where it has been assumed that theconfiguration of certain other (than P-CCPCH) midamble shifts alsoapplies to the other cells of this area. This assumption need not applyif the mobile station examines system information of other cellsdirectly by looking at broadcast information from the other cells, ifpossible, for example from SIB5. In case the system information of acell can be received, it can be confirmed the cell is not a ghost cell,the configuration of midamble shifts can be learned and used for themeasurement of the RSCP.

1) Reduction in Overall Receive Power for Detected Midamble Shift Otherthan to be Detected Midamble Shift

If a configured midamble shift on to be detected cell is detected asidefrom the to be detected (e.g. P-CCPCH) midamble shift, the detectedpower of this shift is subtracted from the overall receive power of themidamble before applying the ghost detection threshold.

In a specific example of this, assume that a cell is near the boundaryto become a ghost cell in presence of an unknown interferer.

For this example, the power on the P-CCPCH is assumed to be about 9 dBlower than the interference. The total received power during themidamble is composed of the following powers in the worst case:

-   -   1. P-CCPCH of to be measured cell: 1 pico Watt—(this value is        arbitrarily chosen for this example)    -   2. Power of interference: 8 pico Watt (9 dB>P-CCPCH)    -   3. Power of the other physical channels on TS0: 2 pico Watt        (twice the power of the P-CCPCH as per worst case assumption)        -   Total receive power: 11=8+1+2 pico Watt            Ghost Cell Detection in dB:            total receive power (db)−threshold (dB)>RSCP (dB)→ghost cell            Ghost Cell Detection in Pico Watt:            Total receive power(pico Watts)/threshold(pico Watt)>RSCP            (Watts)→ghost cell

Assuming that the ghost detection threshold is 10 dB, for the case whereno subtraction of the other physical channels is performed, the test isapplied as follows:Total received power/10(ghost detection threshold: 10 dB*log 10(10)=10dB)=11/10=1.1>RSCP of P-CCPCH(=1)→power is below ghost cell detectionthreshold, so a ghost cell would be detected

Assuming that the ghost detection threshold is 10 dB, for the case wheresubtraction of the other physical channels is performed:

-   -   Total received power—detected powers of the other midamble        shifts used in TS0 of the to be detected cell: 11−2=9    -   Ghost detection threshold: 9/10=0.9<1→power is above ghost cell        detection threshold, so cell is treated as a reliable cell.        2) Adjustments to the Subtraction Amounts

Because the P-CCPCH is transmitted on an omni antenna, and otherchannels may be transmitted using beam forming, every path/tap that ispresent in one of the other channels should also be present in theP-CCPCH. Thus, if additional paths/taps are present on the otherchannels, these additional paths/taps can be treated as noise.

If the set of taps on the P-CCPCH is {SET_A}, and the set of taps onanother channel is {SET_B}, then for the purpose of determining anamount to subtract from the overall receive signal during the preamble,only the taps of {SET_B} that are also in {SET_A} are subtracted.

For example, assume the paths for the midamble shift for the S-CCPCH are1, 3, 8, 10, and the paths for P-CCPCH are 5 to 10. Then the power ofthe S-CCPCH on taps 1 and 3 is set to zero before subtracting an amountfrom the total receive power. This has the effect of making the powerthat is subtracted less, in turn making it more difficult for a ghostcell to stay undetected, and detected as an OK cell.

3) Adjustments Made Based on Power in Unused Windows

The default midamble allocation K=4 leaves half of the channelestimation windows unused (TS0 is configured for the P_CCPCH using twoof the 16 channel estimation windows for channel estimation in order tocover cells with a big delay spread. This leaves only 8 bigger channelestimation windows in TS0. With the K=4 configuration, 4 of them areused leaving the other 4 unused. There may some receive power left afterpost processing in the unused channel estimation windows, and this isnoise. The amount of receive power in the unused channel estimationwindows can be determined.

Recall that an amount is subtracted from the total receive power foreach of S-CCPCH, FPACH and PICH that are present in TS0. Each has itsown channel estimation window. In some embodiments, conditionalsubtraction based on the power of used window relative to power ofunused window is employed as follows:

For each used window (S-CCPCH, FPACH and PICH), if the power in the usedwindow after post processing (sum of the taps) is not larger than the(average) power in the unused estimation windows by a threshold amount(e.g. a factor of 2) then an amount for that window is not subtractedfrom the total receive power for the purpose of ghost cell detection.

In some embodiments, ghost cell detection is also based on power of usedwindow relative to power of unused window. If there is some receivepower left after post processing in the unused channel estimationwindows this is noise. For the P-CCPCH window, if the power in the usedwindow after post processing (sum of the taps) is not larger than thepower in the unused estimation by a threshold amount (e.g. a factor of2) then a ghost cell is detected.

This part of the method is dealing with the detection of noise in theused channel estimation windows. For example, consider a case where thereceive power in the unused channel estimation windows is the same as ina used one. Then this means that the power in the unused channelestimation windows is noise for sure and that the power in the usedchannel estimation windows is noise, most likely, too. Only if thereceive power in the used channel estimation windows is large enough (asdefined by a threshold amount) than the power in the unused channelestimation windows is it considered to carry the power of the usedmidamble shift. Otherwise all the paths in the used channel estimationwindows are set to 0. If the P-CCPCH midamble shift is concerned thisleads to a detected ghost cell.

4) Regression Based Method of Differentiating Between Signalling andNoise.

Raw channel estimates for a to be detected cell, or a to be cancelledcell, are obtained in the form of taps at the outcome of filtering withthe basic midamble code. At this point, no particular midamble shift isyet being applied. The taps are the sorted in descending order ofmagnitude. The sorted taps can be plotted in a logarithmic scale. Theshape of the plot of taps thus generated can be referred to as asignature for a given signal.

A signal that was purely noise, and a signal that was purelyinterference would have the same signature. This signature can beapproximated with a square/square root or linear function. Thisinformation can be used to extract a noise/interference signature fromthe raw channel estimates.

If the sorted raw channel estimates are composed of N taps (e.g. 128taps), it can be assumed that the last M (e.g. 88 taps) of these arenoise/interference only—i.e. they do not contain any component of thesignal of interest. Linear regression on L (e.g. 60 taps) of these lastM taps can be performed to derive a square or linear function describingthe noise/interference signature over L of the last M taps, the last M-Ltaps (e.g. the last 28 taps) are not considered in the regression; theresult can then be extrapolated to the first N taps to produce anestimate of an overall noise/interference signature over the N taps.

Next, a post processing step on the first N-M taps is performed. Inorder for a given tap to be considered reliable, it must exceed thenoise/interference signature for that tap by a tap-wise detectionthreshold amount. If a given tap does not exceed the noise/interferencesignature by this tap-wise threshold amount, it is considered noise, andis set to zero. Then, the remaining taps are returned to their originalorder.

In the case of a to be cancelled cell, these taps are then used forinterference cancellation purposes.

In the case of a to be detected cell, these taps are then used for ghostcell detection purposes. Note that the adjustments described above underone or a combination of items 1) Reduction in overall receive power fordetected midamble shift other than to be detected midamble shift, 2)Adjustments to the subtraction amounts, and 3) Adjustments made based onpower in unused windows, can also be applied, using the set of tapsoutput by the regression approach.

A set of simulations using regressed signature functions was performed.For these examples, the raw channel estimate is composed of N=128 taps,and M=60 of these are used to develop the noise/interference signature.The following parameters were assumed for the simulations:

Tap-Wise Detection Threshold for the to be Detected Midamble: 5 dB

Tap-Wise Detection Threshold for to be Cancelled Midamble: 6 dB

Interference Cancellation Threshold: 6 dB

In the worst case scenario, there is a significant ghost detection ratewithout interference cancellation once the two midambles have the samepower so the threshold can be increased.

Ghost Detection Threshold without IC Applied: 8 dB

This threshold can be lowered because otherwise too many ghost cellswould have stayed undetected. This is a tribute to the toughinterference situation.

Ghost Detection Threshold with IC Applied: 20 dB

Laboratory measurements have shown that with IC ghost cells have beendetected beyond 20 dB power difference. This adjustment has nothing todo with the increased interference problem.

Other Midamble (MA) Shifts: ‘V’ and ‘N’:

Y means that the midamble shifts for S-CCPCH, PICH and FPACH are usedfor both interfering and detected cell.

N means that the midamble shifts for S-CCPCH, PICH and FPACH are notused for both interfering and detected cell.

Used MA Shift str: ‘10101010’—

This means that the detector is assuming that out of the 8 midambleshifts with 16 chip channel estimation length shifts 1, 3, 5, and 7could be used. The other 4 channel estimation windows are assumed not tobe used.

Ghost Detection Threshold 2: 6 dB

In case the used channel estimation window is not at least 6 dB strongerthan the average measured signal strength of the unused windows all thepaths in this window are set to 0.

Lower Ghost Detection Bound: 6 dB

Modified because new algorithms have an adverse effect on theinterference less case.

In the following the results with the regression method are shown forthe best case scenario. The results with the standards deviation areomitted.

FIG. 24 depicts simulation results where there is an unknown interferingcell and no interference cancellation, and a static 1 tap channel isassumed. The result looks fine in the sense that there is a sharpdetection transition zone at ISR of 8-9 dB. It is noted that for theworst case the actual instantaneous receive power for the interferingcell is 5 dB higher—as per worse case assumption. The best case resultsare depicted in the left hand two plots, while the worst case resultsare depicted in the right hand two plots. For the best case, no otherthan P-CCPCH MA shift is used but 4 MA shifts were considered. For theworst case, 2 other than P-CCPCH MA shifts used and 4 MA shifts wereconsidered.

FIG. 25 depicts simulation results where there is a known interferingcell and interference cancellation, and a static 1 tap channel isassumed. The result looks fine. Please note that for the worst case theactual instantaneous RX power for the interfering cell is 5 dB higher—asper worse case assumption. The best case results are depicted in theleft hand two plots, while the worst case results are depicted in theright hand two plots. For the best case, no other than P-CCPCH MA shiftwas used but 4 MA shifts were considered. For the worst case, 2 otherthan P-CCPCH MA shifts were used and 4 MA shifts were considered.

FIG. 26 depicts simulation results where there is an unknown interferingcell and no interference cancellation, and a 2 tap equalizer testchannel is assumed. The result is still fine even though the RXmeasurement gets biased and there is a wider transition zone. Pleasenote that for the worst case the actual instantaneous RX power for theinterfering cell is 5 dB higher—as per worse case assumption. The bestcase results are depicted in the left hand two plots, while the worstcase results are depicted in the right hand two plots. For the bestcase, no other than P-CCPCH MA shift was used but 4 MA shifts wereconsidered. For the worst case, 2 other than P-CCPCH MA shifts were usedand 4 MA shifts were considered.

FIG. 27 depicts simulation results where there is a known interferingcell and interference cancellation, and a 2 tap equalizer test channelis assumed. The result still looks fine even though for the worst casebias is to be seen and there are little issues for the ghost detectionto be seen where both cells have the same power. Here the to be detectedcell is also interfering the measurements for the interfering cell. itis noted that for the worst case the actual instantaneous RX power forthe interfering cell is 5 dB higher—as per worse case assumption. Thebest case results are depicted in the left hand two plots, while theworst case results are depicted in the right hand two plots. For thebest case, no other than P-CCPCH MA shift was used but 4 MA shifts wereconsidered. For the worst case, 2 other than P-CCPCH MA shifts were usedand 4 MA shifts were considered.

FIG. 28 depicts simulation results where there is an unknown interferingcell and no interference cancellation, and a 4 tap equalizer testchannel is assumed. The result still looks fine even though for theworst case bias is to be seen and there are little issues for the ghostdetection to be seen where both cells have the same power here the to bedetected cell is also interfering the measurements for the interferingcell. The isolated measurement glitches have to be filtered by postprocessing in order not to cause unwanted events 2 a for example. It isnoted that the worst case the actual instantaneous RX power for theinterfering cell is 5 dB higher—as per worse case assumption. Note alsothat this case is unlikely. The best case results are depicted in theleft hand two plots, while the worst case results are depicted in theright hand two plots. For the best case, no other than P-CCPCH MA shiftwas used but 4 MA shifts were considered. For the worst case, 2 otherthan P-CCPCH MA shifts were used and 4 MA shifts were considered.

FIG. 29 depicts simulation results where there is a known interferingcell and interference cancellation, and a 4 tap equalizer test channelis assumed. The result still looks fine even though for the worst casebias is to be seen and there are little issues for the ghost detectionto be seen where both cells have the same power. Here the to be detectedcell is also interfering the measurements for the interfering cell. Itis noted that for the worst case the actual instantaneous RX power forthe interfering cell is 5 dB higher—as per worse case assumption. Thebest case results are depicted in the left hand two plots, while theworst case results are depicted in the right hand two plots. For thebest case, no other than P-CCPCH MA shift was used but 4 MA shifts wereconsidered. For the worst case, 2 other than P-CCPCH MA shifts were usedand 4 MA shifts were considered.

FIG. 30 depicts simulation results where there is an unknown interferingcell and no interference cancellation, and a 4 tap equalizer testchannel for detected cell and 1 tap for interfering cell were assumed.The result shows that ghost cells can be detected at a quite low ISRalready. It is noted that for the worst case the actual instantaneous RXpower for the interfering cell is 5 dB higher—as per worse caseassumption. The best case results are depicted in the left hand twoplots, while the worst case results are depicted in the right hand twoplots. For the best case, no other than P-CCPCH MA shift was used but 4MA shifts were considered. For the worst case, 2 other than P-CCPCH MAshifts were used and 4 MA shifts were considered.

FIG. 31 depicts simulation results where there is a known interferingcell and interference cancellation, and a 4 tap equalizer test channelfor detected cell and 1 tap for interfering cell were assumed. Theresult looks fine. It is noted that for the worst case the actualinstantaneous RX power for the interfering cell is 5 dB higher—as perworse case assumption. The best case results are depicted in the lefthand two plots, while the worst case results are depicted in the righthand two plots. For the best case, no other than P-CCPCH MA shift wasused but 4 MA shifts were considered. For the worst case, 2 other thanP-CCPCH MA shifts were used and 4 MA shifts were considered.

FIG. 32 depicts simulation results where there is an unknown interferingcell and no interference cancellation, and an 8 tap equalizer testchannel is assumed. The result looks worse because in the worst caseghost cells are detected at a very low ISR. It is noted that for theworst case the actual instantaneous RX power for the interfering cell is5 dB higher—as per worse case assumption. This case is very unlikely.The best case results are depicted in the left hand two plots, while theworst case results are depicted in the right hand two plots. For thebest case, no other than P-CCPCH MA shift was used but 4 MA shifts wereconsidered. For the worst case, 2 other than P-CCPCH MA shifts were usedand 4 MA shifts were considered.

FIG. 33 depicts simulation results where there is a known interferingcell and interference cancellation, and an 8 tap equalizer test channelis assumed. The result looks bad for the worst case in scenarios likethis (however unlikely they are) it is least advisable to load 1 TS with3 omni cell channels. It is noted that for the worst case the actualinstantaneous RX power for the interfering cell is 5 dB higher—as perworse case assumption. The best case results are depicted in the lefthand two plots, while the worst case results are depicted in the righthand two plots. For the best case, no other than P-CCPCH MA shift wasused but 4 MA shifts were considered. For the worst case, 2 other thanP-CCPCH MA shifts were used and 4 MA shifts were considered.

FIG. 34 depicts simulation results where there is AWGN and noInterference. The result looks fine. As expected there is no significantdifference in-between best and worst case. The best case results aredepicted in the left hand two plots, while the worst case results aredepicted in the right hand two plots. For the best case, no other thanP-CCPCH MA shift was used but 4 MA shifts were considered. For the worstcase, 2 other than P-CCPCH MA shifts were used and 4 MA shifts wereconsidered.

FIG. 35 depicts simulation results where a static 8 tap equalizer testchannel is assumed and there is interference. The result looks fine. Forthe ghost detection rate there is a difference in-between best and worstcase. The best case results are depicted in the left hand two plots,while the worst case results are depicted in the right hand two plots.For the best case, no other than P-CCPCH MA shift was used but 4 MAshifts were considered. For the worst case, 2 other than P-CCPCH MAshifts were used and 4 MA shifts were considered.

FIG. 36 depicts simulation results where a fading 8 tap equalizer testchannel is assumed and there is no interference. The result looks fine.As expected there is no significant difference in-between best and worstcase. The best case results are depicted in the left hand two plots,while the worst case results are depicted in the right hand two plots.For the best case, no other than P-CCPCH MA shift was used but 4 MAshifts were considered. For the worst case, 2 other than P-CCPCH MAshifts were used and 4 MA shifts were considered.

FIG. 37 depicts simulation results where Rayleigh fading and nointerference are assumed. The result looks fine. As expected there is nosignificant difference in-between best and worst case. The best caseresults are depicted in the left hand two plots, while the worst caseresults are depicted in the right hand two plots. For the best case, noother than P-CCPCH MA shift was used but 4 MA shifts were considered.For the worst case, 2 other than P-CCPCH MA shifts were used and 4 MAshifts were considered.

If TS0 is used for the P-CCPCH alone the described algorithms are wellable to cope with the situation and provide good measurements and gooddetection of the ghost cells at that same time.

Once TS0 is shared with other physical channels the complexity of thealgorithm needs to be stepped up sacrificing some performancenevertheless.

The P-CCPCH of other cells may be able to be detected in interferencescenarios which are just providing enough SNR to detect the BCCH of thatcell reasonably successful. What that SNR is required is implementationspecific. Some embodiments are using the detection of the BCCH to helpin making ghost cell decision.

In some embodiments, every cell should have two states reliable_cell orghost_cell. To transit in-between the two states should is not triggeredby a single measurement but by some criteria applied to multipleconsecutive measurements. For example, some number of consecutivemeasurements, or a majority of some number of consecutive measurements(e.g. 9) must be indicative of state change in order to consider thechange to have occurred.

Depending on the information of the cell make up in SIB5, it can beconcluded what midamble shifts to expect and what not. Dependent on thatthe algorithm can be parameterized differently in order to give a betterperformance.

A ghost cell will have random positions for the strongest paths. It willbe difficult to synchronize. In some embodiments, such information isalso used to aid the identification of a ghost cell.

Since the synchronization to a ghost cell is difficult, in someembodiments, a ghost cell is searched from time to time.

In general the results assuming IC are better than the ones not usingit. In some embodiments, when a ghost cell is detected with a high totalreceive power and no IC being performed, a systematic search for cellsnot included in the neighbor cell list is conducted. This might not needto be done with the full blown cell search algorithm. If the midamble ofthat cells is detected an offset of +/−60 chip the cell can still berecognized from the channel estimate. See the following figure. The RSCPmeasurement algorithm is designed anyway in a way such that with asingle burst multiple estimates can be performed. Some of those could beused for looking for neighbor cells not being in the neighbor cell list.Once a candidate is found, the position is confirmed with a more precisealgorithm and then the interferer not in the neighbor cell list can beused to create an unbiased estimation of the RSCP of the cells in theneighbor cell list using IC.

Expressed another way, in some embodiments, the channel estimation inputfor one cell may be used as an input to the channel estimation foranother cell. The synchronization of the other cell needs not to beknown exactly. If the input signal for the channel estimation does notcoincide with the location of the midamble in the other cell, thefollowing will be the results:

1. The channel estimate will be shifted. For example, if the midamble ofthe other cell comes in with 20 chip delay compared to the cell thedetection was originally intended, for then the positions of the taps inthe channel estimate cyclically shifts by 20 chip. That means a tap atthe position 5 of the original cell would be at position 25 and a tapbeing at position 128 of the original cell would be at position 20.2. The noise floor in the channel estimate is increasing. The reason isthat also signal portions of the other cell which are not its midambleare in the input signal for the channel estimation. This isself-interference then.

Once there is a strong signal signature in the channel estimate either:

1. The timing is corrected. In a first example, in the case describedbefore correction is achieved by assuming that the tap at position 25 isbelonging to the P-CCPCH and has to be at position 5. Then the timing iscorrected such that the processing is taking place 20 chips later. Inanother example, a tap on position 120 is assumed to belong to theP-CCPCH as well and should be on position 5. In this case, timing wouldbe corrected such that processing is taking place 13 chip earlier.

In case the other cell has multiple midamble shifts in use the resultingambiguity may be resolved by a try and error method.

After that, the synchronization of the other cell will be known, and inthe case of a big timing adjustment the noise floor in the estimateshould decrease.

2. The timing for the other cell is established with a cell searchalgorithm from scratch.

More generally, the search methods described may be used to search forunknown interfering calls in any case. FIG. 38 shows an example of rawchannel estimation output with various misalignments of the midamble.For FIG. 38, it has been assumed that 3 MA shifts are used and that theburst outside the MA areas is filled with AWGN of same power as the MA.

In case the SNR on that carrier would not allow the BCCH of the cells inthe neighbor cell list to be detected with sufficient quality this cellis either not reported or reported at the option of the mobile device(for neighbor cell measurements not being in need to be reported) orreported with the minimum reportable RSCP (for neighbor cellmeasurements which have to be reported). In case the cell has to bereported e.g. for periodic measurements it should be treated with theminimum reportable RSCP. This would provide the best performance for theghost cell detection.

Numerous modifications and variations of the present disclosure arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the disclosuremay be practiced otherwise than as specifically described herein.

For example, while many of the methods and apparatuses have beendescribed in the context of cell reselection, more generally, any ofthese methods can be employed in any situation where cell measurementsare made, for example when making decisions regarding cell selection,cell reselection, handover, etc.

The invention claimed is:
 1. A method comprising: processing a signal toproduce an interference cancellation component of a first cell;performing signal detection for a to be detected cell operating on asame frequency as the first cell based on the signal minus theinterference cancellation component if the interference cancellationcomponent has a power that is large enough compared to a total power ofthe signal as defined by a first threshold; performing signal detectionfor the to be detected cell based on the signal without havingsubtracted the interference component if the interference cancellationcomponent has a power that is not large enough compared to the totalpower of the signal as defined by the first threshold; whereinprocessing the signal to produce the interference cancellation componentcomprises: generating a channel estimate for the signal using acell-specific code of the first cell, the channel estimate comprising aplurality of taps; removing certain taps from the channel estimate toproduce a post-processed channel estimate; using the cell-specific codeand the channel estimate with the certain taps removed to reconstructthe interference cancellation component; wherein performing signaldetection of the to be detected cell based on the signal minus theinterference cancellation component comprises using a cell-specific codeof the to be detected cell; and performing signal detection of the to bedetected cell based on the signal without having subtracted theinterference component comprises using the cell-specific code of the tobe detected cell; wherein removing certain taps comprises removing tapsthat each have a power that is small enough compared to the total powerof the signal as defined by a second threshold.
 2. The method of claim 1wherein each of the taps other than the certain taps has a power that islarge enough compared to the average power of taps other than thecertain taps as defined by a second threshold.
 3. A method comprising:processing a signal to produce a channel estimate in respect of a firstcell, the channel estimate comprising a plurality of taps; removingcertain taps from the channel estimate to leave remaining taps and thenproducing an interference cancellation component from the remainingtaps; performing first signal detection of a to be detected celloperating on a same frequency as the first cell based on the signalminus the interference cancellation component to produce a first signaldetection result; performing second signal detection of the to bedetected cell based on the signal without having subtracted theinterference cancellation component to produce a second signal detectionresult; selecting between the first signal detection result and thesecond signal detection result; wherein: performing first signaldetection comprises performing channel estimation to produce a firstchannel estimate comprising a plurality of taps and removing certaintaps to produce a first post-processed channel estimate; performingsecond signal detection comprises performing channel estimation toproduce a second channel estimate comprising a plurality of taps andremoving certain taps to produce a second post-processed channelestimate; selecting between the first signal detection result and thesecond signal detection result is based on: the power of taps in thefirst post-processed channel estimate; the power of taps in the secondpost-processed channel estimate; the average power of taps in the firstchannel estimate; and the average power of taps in the second channelestimate.
 4. The method of claim 3 wherein selecting comprises: for thefirst signal detection, determining a ratio of a sum of powers of thetaps remaining after removing certain taps to the average power of tapsof the first channel estimate; for the second signal detection,determining a ratio of a sum of powers of the taps remaining afterremoving certain taps to the average power of taps of the second channelestimate; selecting the result with the larger ratio.
 5. A methodcomprising: processing a signal to produce an interference cancellationcomponent that is an estimate of a component of the signal that is dueto a first cell by: generating a channel estimate for the signal using acell-specific code of the first cell to produce a channel estimatecomprising a plurality of taps; removing certain taps from the channelestimate to produce a post-processed channel estimate; producing theinterference cancellation component using post-processed channelestimate; performing channel estimation of a to be detected celloperating on a same frequency as the first cell based on the signalminus the interference cancellation component by: generating a channelestimate for the to be detected cell using a cell-specific code of theto be detected cell; removing certain taps from the channel estimate toproduce a channel estimate with certain taps removed for the to bedetected cell; the method further comprising at least one of: a)discarding the channel estimate with certain taps removed for the to bedetected cell and/or reporting a lowest reportable value and/orprocessing a very small value and/or not reporting the channel estimateif a combined power of the taps of the channel estimate with certaintaps removed that define a to be measured channel for the to be detectedcell is below a total receive power for the signal by a thresholdamount; and b) discarding the channel estimate with certain taps removedfor the to be detected cell and/or reporting a lowest reportable valueand/or processing a very small value and/or not reporting the channelestimate if a combined power of the taps of the channel estimate withcertain taps removed for the first cell is small enough compared to atotal receive power for the signal as defined by a threshold amount. 6.The method of claim 5 wherein using a regression approach comprises:sorting the plurality of taps into a sorted list; performing aregression on a subset of taps in a sorted list representinginterference and noise to produce a regression result; removing tapsthat are not large enough compared to the regression result as definedby a threshold.
 7. A method according to claim 5, comprising: withoutfirst performing interference cancellation, a mobile device processingthe signal to detect a ghost cell, the ghost cell being a cell with anunreliable measurement result as defined by ghost cell detectioncriteria; in case the ghost cell is detected, the mobile devicesystematically searching for cells and their midamble codes which arenot in a neighbor cell list; for each cell found as a result of thesystematic search, the wireless device applying interferencecancellation of a component of the received signal due to the cell if atotal received power as defined by a sum of taps of a post processedchannel estimate of the cell is large enough as defined by a thresholdcompared to the overall received power of the midamble.
 8. A methodaccording to claim 5, comprising: processing the signal to detect a cellin accordance with a ghost cell detection criteria, a ghost cell being acell with an unreliable measurement result as defined by the ghost celldetection criteria; in case a cell is detected with the ghost detectionthreshold, determining that the cell is a ghost cell if the cell's BCCH(broadcast control channel) cannot be detected; determining that thecell is not a ghost cell if the cell's BCCH can be detectedsuccessfully.
 9. The method of claim 5 further comprising: determiningif a cell measurement is reliable or unreliable in accordance with acriteria that spans over multiple measurement intervals.
 10. The methodof claim 5 further comprising: processing the signal to detect a cell inaccordance with a ghost cell detection criteria, a ghost cell being acell with an unreliable measurement result as defined by the ghost celldetection criteria; looking systematically for cells that are notincluded in a neighbor cell list, and if some are found, treating themas known cells, for the purpose of performing interference cancellation;at least one of: if no such cell is found, then declaring the celldetected in accordance with the ghost detection criteria to be a ghostcell; if no such cell is found, attempting to detect a BCCH of the cell,and declaring the cell detected in accordance with the ghost detectioncriteria to be a ghost cell if the BCCH cannot be detected.
 11. A methodof claim 5 further comprising: assigning one of two states to a cell,the two states being reliable_cell or ghost_cell; transitioning betweenthe two states on the basis of a criteria applied to multipleconsecutive measurements.
 12. A method of claim 5 further comprising:recognizing when it is difficult to synchronize to a cell, and usingsuch information to aid the identification of a ghost cell.
 13. A methodof claim 5 further comprising: when a ghost cell is detected with a hightotal receive power and no interference cancellation being performed,performing a systematic search for cells not included in a neighbor celllist, and if some are found treating them as known cells for the purposeof interference cancellation.
 14. A method of processing a signal, themethod comprising: obtaining a channel estimate for a cell, the channelestimate comprising a plurality of taps; removing certain taps from thechannel estimate; if a combined power of taps of the channel estimateafter removing certain taps that define a to be measured channel is lessthan a certain threshold below an amount based on a total power of thesignal, discarding the channel estimate for the cell and/or reporting aminimum reportable receive power and/or processing a very small value;wherein removing certain taps from the channel estimate comprises oneof: using a regression approach to differentiate between the taps to beremoved and the taps not to be removed; and removing taps that each havea power that is small enough compared to the total power of the signalas defined by a second threshold.
 15. The method of claim 14 whereintaps other than the certain taps are those that each have a power thatis large enough compared to the average power of the certain taps asdefined by a second threshold.
 16. A method comprising: without firstperforming interference cancellation, a mobile device processing asignal to detect a ghost cell, the ghost cell being a cell with anunreliable measurement result as defined by ghost cell detectioncriteria; in case the ghost cell is detected, the mobile devicesystematically searching for cells and their midamble codes which arenot in a neighbor cell list; for each cell found as a result of thesystematic search, the wireless device applying interferencecancellation of a component of the received signal due to the cell if atotal received power as defined by a sum of taps of a post processedchannel estimate of the cell is large enough as defined by a thresholdcompared to the overall received power of the midamble.